The proposed research will relate naturally occurring patterns of communication in early infant-caregiver social interaction to developmental outcomes in the infant siblings (High Risk infants; HR) of children with autism spectrum disorder (ASD)-a population at heightened risk for developing ASD as well as significant language delays (Gamliel, Yirmiya, & Sigman, 2007; Stone, McMahon, Yoder, & Walden, 2007; Toth, Dawson, Meltzoff, Greenson, & Fein, 2007). Interactions between infants and their caregivers are fundamental to social learning and language development (e.g. Tamis-LeMonda, Bornstein, & Baumwell, 2001, Landry, Smith, Miller Loncar, and Swank, 1997), yet this important developmental context has been understudied in the HR sibling literature. A transactional model of development suggests that infants learn through a dyadic process that involves both their own behavior and the input they receive from adults around them (Sameroff, 2009). A disruption in this dyadic process could therefore have cascading effects in multiple modalities, making social interactions an ideal context in which to understand both typical and atypical development. Using time-locked micro-coding of parent and infant gaze, speech, and gesture production, along with sophisticated data analytic techniques for describing the coordination of dyadic behavior, this research will: 1) Describe differences in the production and coordination of parent and infant communicative behaviors during naturalistic interactions at 6, 9 , and 12 months between low risk dyads (LR; no first degree relatives with ASD) and three outcome groups of HR dyads: HR dyads with infants without delays (HR-ND), HR dyads with infants with significant language delays (HR-LD), and HR dyads with infants with autism spectrum disorder (HR-ASD) and 2) Analyze the predictive utility of measures of coordination to language outcomes in toddlerhood. The primary goals for this fellowship training involve developing expertise in the theory, empirical research, methodology, and quantitative skills necessary to understand and study parent-infant interactions and their relationship to development. In the context of this training, the applicant will learn advanced techniques for coding and analyzing parent and infant communicative behavior that will allow for a more sophisticated understanding of this complex, bidirectional interactions and their relationship to language development. This work is intended to address the NIDCD's strategic objectives of improving diagnosis of individuals at risk for language and communication disorders and understanding language acquisition in ASD specifically. Findings from the proposed research have the potential to uncover early markers and mechanisms of delays and deficits in language and communication abilities in a high risk population and to inform early interventions.